Osteoporosis encompasses a broad range of clinical syndromes having varying etiologies. In postmenopausal women, for example, two distinct types of osteoporosis have been identified. Type I osteoporosis occurs mainly in the early postmenopausai period from about age 50-65. It is characterized by excessive resorption, primarily in trabecular bone. Vertebral fractures are common and if given prior to significant bone loss, treatment which decreases or prevents bone resorption (such as estrogen or calcitonin) is considered effective therapy.
Type II osteoporosis (a.k.a. senile osteoporosis) occurs essentially in all aging women and, to a lesser extent, in men. It is characterized by proportionate loss of conical and trabecular bone. Here decreased bone formation plays a major role, if not a more important role than increased bone resorption. Fractures of the hip are characteristic of this type.
Currently approved therapeutic agents for osteoporosis are antiresorptives. As such, they are not as effective in patients with established osteoporosis of either type (decreased bone density with fractures of the vertebrae and/or hip), or in patients with Type II osteoporosis. In addition, the most accepted preventive agent for osteoporosis currently in use is estrogen therapy, which is not really an acceptable therapeutic agent for women with a history of breast cancer or endometrial cancer or for men with osteoporosis.
Insulin-like Growth Factor I (IGF-I) is a 70 amino acid peptide belonging to a family of compounds under the class name somatedins and retains structural and biological similarities to insulin. The somatedins' activity lie on a spectrum from hypoglycemic effects similar to insulin to growth promoting effects which are exemplified by growth hormone. IGF-I predominantly induces growth and cell proliferation. IGF-I has also been demonstrated to specifically bind to receptors on rat osteoblast-like bone cells (Bennett et al, Endocrin. 115 (4): 1577-1583, 1984). IGF-I is routinely fabricated in the liver and released for binding to carrier proteins in the plasma (Schwander et al, Endocrin. 113 (1): 297-305, 1983), which bound form is inactive. In addition, there is a biofeedback regulating loop involving the somatomedins and growth hormone such that higher somatomedin concentrations inhibit growth hormone release which results in lesser production of endogenous IGF-I.
IGF-I infused into rats has been shown to result in markedly greater increases in body weight gain compared to controls, with increases in tibial epiphyseal width and thymidine incorporation into costal cartilage (Nature 107: 16-24, 1984) and directly stimulate osteoblasts to result in a greater number of functional osteoblasts. IGF-I is also mentioned as the vehicle through which growth hormone's effects on bone is mediated in Simpson, Growth Factors Which Affect Bone, Physiol. 235, TIBS, 12/84.
Nevertheless, it is important to note that the foregoing pre-clinical studies were conducted with fetal or newborn rat cells. It is highly likely that such "young" cells are more responsive to IGF-I (as well as other influences) than older cells, especially those in the elderly with established osteoporosis or those with drug or environmentally induced defects leading to reduced bone density.
Surprisingly, IGF-I has now been found to be useful in the treatment of osteoporosis in mammals exhibiting decreased bone mineral density and those exposed to drugs or environmental conditions which tend to result in bone density reduction and potentially to an osteoporosis condition.
Accordingly, an object of the present invention is to provide a method of treatment of osteoporosis in mammals exhibiting decreased bone mineral density and preventing osteoporosis due to bone mineral density reduction in patients who are clinically prone to such bone mineral density reductions.
Another object of the invention is to provide pharmaceutical compositions useful in achieving the foregoing object.